Power units are required that can provide economical power generation in many situations, including those with wide demand fluctuations. And, pollution control measures must be available to operate effectively despite these fluctuations.
Demand for power can vary over time because of localized issues or because of seasonal changes. When demand fluctuates for an individual unit supplying power to a generator, the low demand operations will generally produce lower effluent temperatures than operations at high load. Demand often fluctuates in the summer when air conditioning adds to the demand of industrial uses. At night, demand will typically be lower than during the day; but because NOx is generally a problem whenever air conditioning is in demand, even the relatively lower NOx production at night must be effectively addressed.
Unfortunately, while NOx reduction technologies have been developed to the extent that air pollution can be effectively controlled regardless of the season, the wide load fluctuations that accompany heavy seasonal demand, can stress the systems. The problem is partly due to the sheer volume of NOx being generated and partly due to the temperature sensitivity of SNCR (selective non-catalytic reduction) systems and SCR (selective catalytic reduction) systems. Both are limited in effectiveness to specific temperature windows, and the flue gases generated by regulated combustors must be treated within those windows.
SCR has been proven to be highly effective for NOx reduction, and SCR units can generally be scaled to the size required for peak generation units, such as package boilers Diesel engines, turbines and the like. However, SCR units typically require the use of ammonia as a reducing reagent, and it is a common problem that ammonia is difficult and dangerous to store, especially in populated areas. Advantageously, urea gasification units such as described in U.S. Pat. No. 7,090,810 to Sun, et al., can be effectively employed, but their control during fluctuating load can be a serious problem. These systems may draw process gases as ambient air or as combustion gases from the combustor, typically from a single location where the gas temperature is desirably economical for gasification. The effluent should be hot enough to achieve full gasification with minimal heating, but low loads produce temperatures lower than required for optimal operation.
Once gasified, the gases produced are typically fed to the SCR units by means of ammonia injection grids (AIGs), which are essentially arrays of distribution pipes with holes arranged through which the ammonia is preferably ejected. Where demand is low, the volume of gasified urea will also decrease and will not, by itself, be sufficient to provide sufficient momentum for the gas and thereby achieve uniform distribution for NOx reduction.
There is a particular need for a process and an apparatus which convert urea to an ammonia-containing gas, yet maintain the ability to fully control the operation of a SCR unit regardless of demand for power.